How do I automatically determine the best commit to interactively rebase from?

I frequently use "fixup!", "squash!" and "rebase -i" to clean up, correct, or just fiddle with recent, unpushed history. I'd like to find a way to get Git to automatically choose which commit makes the most sense for interactive rebasing, so that I could define an alias to do a quick interactive rebase.

I think the best commit in such cases would be the most recent ancestor to either:

appear in any other branch (local or remote)

have more than one parent

have no parents

The first condition ensures that I don't rewrite commits that might affect others. The second ensures that I don't try to rebase "through" a merge, which is definitely not what I want. The third is just a sanity check.

I'm sure there's a simpler, more robust way of accomplishing this, but it seems to do what I want. Suggestions and improvements are very much welcome. (It'd be nice to do this with just Git plumbing instead of relying on wc, for example.)